1. Field of the Invention
This invention relates to a roof structure of the type using a plurality of roofing panels, adjacent margins of which are form-coupled to provide a functionally continuous roof, and more particularly to such a roof structure which incorporates rigid board insulation.
2. Description of the Prior Art
Typical pre-engineered metal building roof structures such as the structure disclosed in U.S. Pat. No. 4,102,105, include a plurality of spaced purlins and a series of pairs of relatively stiff and rigid interlocked roof panels enclosing the space between the spaced purlins. Concealed fasteners are generally utilized to secure the pairs of roof panels to the underlying purlins. The panels are almost always lapped and sealed, depending upon the length of the roof and the panel length. Typically, the entire roof system generally utilizes a single layer blanket thermal insulation, such as flexible glass fiber blankets, laid end-to-end and stretched over the spaced purlins. This insulation is installed just prior to insulation of the roof panels.
The insulation is pinched or compressed at each purlin location when the roof panels are fastened to the purlins. The compressed or distorted insulation forms a "thermal weakness" or heat path having low thermal resistance. Accordingly, much heat loss can occur through the insulated roof at and near each purlin location. When it is considered that purlins are spaced generally at five foot intervals and that the width of a purlin flange is three inches, it will readily be seen that there is a substantial heat loss.
Aside from the "thermal weakness" areas near purlins, blanket type insulation is inadequate for many building thermal requirements. Using increased thickness or multiple layers of thicknesses are frequently still inadequate and inefficient.
The prior art has sought to minimize the heat loss by utilizing "thermal spacers" at purlins. A further solution for overcoming thermal inefficiency is proposed in U.S. Pat. No. 4,058,949. This patent suggests using a plurality of layers of flexible insulation. The layers of insulation are separated by U-shaped channel members at the purlin location. However, this has not proven to be a satisfactory solution to the problem. The insulation is still pinched or distorted between the roofing panels, U-shaped member, and purlins. The same "thermal weakness", although possibly to a lesser degree, still exists. Even without the distorted insulation, blanket insulation is inefficient. It has been determined more than six inches of blanket insulation is required to equal the thermal efficiency of three inches of rigid board insulation. But, only about four inches maximum blanket insulation is thermally efficient.
Accordingly, there is a real need to develop better insulation systems. Most present roof systems use concealed fasteners. Rigid board insulation does not readily accommodate concealed fasteners.